feat: support freshness parameter for Perplexity web_search provider (#15343)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 01aba2bfba053c028f62970dcd153b19d00c5e3b
Co-authored-by: echoVic <16428813+echoVic@users.noreply.github.com>
Co-authored-by: sebslight <19554889+sebslight@users.noreply.github.com>
Reviewed-by: @sebslight

docs/tools/web.md

@@ -175,7 +175,9 @@ Search the web using your configured provider.
 - `country` (optional): 2-letter country code for region-specific results (e.g., "DE", "US", "ALL"). If omitted, Brave chooses its default region.
 - `search_lang` (optional): ISO language code for search results (e.g., "de", "en", "fr")
 - `ui_lang` (optional): ISO language code for UI elements
-- `freshness` (optional, Brave only): filter by discovery time (`pd`, `pw`, `pm`, `py`, or `YYYY-MM-DDtoYYYY-MM-DD`)
+- `freshness` (optional): filter by discovery time
+  - Brave: `pd`, `pw`, `pm`, `py`, or `YYYY-MM-DDtoYYYY-MM-DD`
+  - Perplexity: `pd`, `pw`, `pm`, `py`
 
 **Examples:**